Revamp the Readme of the parachain template (#4713)

- Addresses
[this](https://github.com/paritytech/polkadot-sdk/issues/3155#issuecomment-2126934939).
- Revamps the Readme, very similar to [the minimal
template](https://github.com/paritytech/polkadot-sdk/pull/4649).
- Changed `polkadot-launch` to `zombienet`, with instructions how to run
it.
- See the [rendered
version](https://github.com/paritytech/polkadot-sdk/blob/rzadp/parachain-template-readme/templates/parachain/README.md).